The invention relates generally to communication systems, and more specifically to an architecture for a SONET line unit.
As it is generally known, SONET (Synchronous Optical Network) defines a set of standards for a synchronous optical hierarchy that has the flexibility to transport many digital signals having different capacities. A corresponding international synchronous digital hierarchy (SDH) standard provides a set of definitions analogous to those of SONET. The synchronous nature of SONET is provided by a receive side and a transmit side clock in each network element (NE). In order to synchronize the receive and transmit clocks, a SONET network element, such as an add-drop multiplexer, includes circuitry to recover clock signals from various sources that may be available, and to distribute highly accurate clocks internally based on such recovery.
A central timing source provides a Building Integrated Time Source, also referred to as a "BITS" clock, that may be provided out-of-band to each network element in a SONET ring. If a network element is for some reason not able to receive the BITS clock directly, an embedded clock may be recovered by that device from an incoming line that should reflect the centrally provided BITS clock.
The basic building block in SONET is a synchronous transport signal level-1 (STS-1), which is transported as a 51.840-Mb/s serial transmission using an optical carrier level-1 (OC-1) optical signal. Higher data rates are transported using SONET by multiplexing N lower level signals together. To this end, SONET defines optical and electrical signals designated as OC-N (Optical Carrier level-N) and STS-N (Synchronous transport signal level-N), where OC-N and STS-N have the same data rate for a given value of N. Accordingly, just as STS-1 and OC-1 share a common data rate of 51.84 Mb/s, OC-3/STS-3 both have a data rate of 155.52 Mb/s.
Information transported via an STS-1 signal is organized as frames, each having 6480 bits (810 bytes). An STS-1 frame includes transport overhead and a Synchronous Payload Envelope (SPE). The SPE includes a payload, which is typically mapped into the SPE by what is referred to as path terminating equipment at what is known as the path layer of the SONET architecture. Line terminating equipment, such as an OC-N to OC-M multiplexer, is used to place an SPE into a frame, along with certain line overhead (LOH) bytes. The LOH bytes provide information for line protection and maintenance purposes. The section layer in SONET transports the STS-N frame over a physical medium, such as optical fiber, and is associated with a number of section overhead (SOH) bytes. The SOH bytes are used for framing, section monitoring, and section level equipment communication. Finally, a physical layer in SONET transports the bits serially as either electrical or optical entities.
The SPE portion of an STS-1 frame is contained within an area of an STS-1 frame that is typically viewed as a matrix of bytes having 87 columns and 9 rows. Two columns of the matrix (30 and 59) contain fixed stuff bytes. Another column contains STS-1 POH. The payload of an SPE may have its first byte anywhere inside the SPE matrix, and, in fact may move around in this area between frames. The method by which the starting payload location is determined is responsive to the contents of transport overhead bytes in the frame referred to as H1 and H2. H1 and H2 store an offset value referred to as a "pointer", indicating a location in the STS-1 frame in which the first payload byte is located.
The pointer value enables a SONET network element to operate in the face of certain conditions which may, for example, cause the STS-1 frame rate to become faster or slower than the SPE insertion rate. This situation may arise when the clock of the NE must be derived from a relatively less accurate clock source, in order to continue operation, when a more accurate source, such as the BITS clock itself, has been lost. In such a case, an extra byte may need to be transmitted in what is known as a negative justification opportunity byte, or, one less byte may be transmitted in a given STS-1 frame so as to accommodate the SPE, thus causing the location of the beginning of the payload to vary.
Various digital signals, such as those defined in the well-known Digital Multiplex Hierarchy (DMH), may be included in the SPE payload. The DMH defines signals including DS-0 (referred to as a 64-kb/s time slot), DS-1 (1.544 Mb/s), and DS-3 (44.736 Mb/s). The SONET standard is sufficiently flexible to allow new data rates to be supported, as services require them. In a common implementation, DS-1s are mapped into virtual tributaries (VTs), which are in turn multiplexed into an STS-1 SPE, and are then multiplexed into an optical carrier-N (OC-N) optical line rate.
The payload of a particular SPE may be associated with one of four different sizes of virtual tributaries (VTs). The VTs are VT1.5 having a data rate of 1.728 Mb/s, VT2 at 2.304 Mb/s, VT3 at 3.456 Mb/s, and, VT6 at 6.912 Mb/s. A superframe consists of four STS-1 frames, and is used to transmit a VT. The alignment of a VT within the bytes of the payload allocated for that VT is indicated by a pointer contained within two VT pointer bytes, which contain a pointer offset similar to the STS-1 pointer described above.
Existing add-drop multiplexers (ADMs) are SONET multiplexers that allow DS-1 and other DMH signals to be added into or dropped from an STS-1 signal. Traditional ADMs have two bi-directional ports, and may be used in self-healing ring (SHR) network architectures. An SHR uses a collection of network elements including ADMs in a physical closed loop so that each network element is connected with a duplex connection through its ports to two adjacent nodes. Any loss of connection due to a single failure of a network element or a connection between network elements may be automatically restored in this topology. Existing ADMs have additionally included a cross-connect matrix for directing STM signals from one interface to another. Such a cross-connect matrix is referred to as an STM switch fabric. The manner in which specific STM signals are directed between interfaces of the STM switch fabric depends on how the network bandwidth has been "provisioned" to the various customers using the network. The path of a signal through a given cross-connect matrix is statically defined based on provisioning information provided from a central office or "craft" technician.
As mentioned above, SONET provides substantial overhead information. SONET overhead information is accessed, generated, and processed by the equipment which terminates the particular overhead layer. More specifically, section terminating equipment operates on nine bytes of section overhead, which are used for communications between adjacent network elements. Section overhead supports functions such as: performance monitoring (STS-N signal), local orderwire, data communication channels (DCC) to carry information for OAM&P, and framing. The section overhead is found in the first three rows of columns 1 through 9 of the SPE.
Line terminating equipment operates on line overhead, which is used for the STS-N signal between STS-N multiplexers. Line overhead consists of 18 overhead bytes, and supports functions such as: locating the SPE in the frame, multiplexing or concatenating signals, performance monitoring, automatic protection switching, and line maintenance. The line overhead is found in rows 4 to 9 of columns 1 through 9 of the SPE.
Path overhead bytes (POH) are associated with the path layer, and are included in the SPE. Path-level overhead, in the form of either VT path overhead or STS path overhead, is carried from end-to-end; it is added to DS1 signals when they are mapped into virtual tributaries and for STS-1 payloads that travel end-to-end. VT path overhead (VT POH) terminating equipment operates on four evenly distributed VT path overhead bytes starting at the first byte of the VT payload, as indicated by the VT payload pointer. VT POH provides communication between the point of creation of an VT SPE and its point of disassembly.
STS path terminating equipment terminates STS path overhead (STS POH) consisting of nine evenly distributed bytes starting at the first byte of the STS SPE. STS POH provides for communication between the point of creation of an STS SPE and its point of disassembly. STS path overhead supports functions such as: performance monitoring of the STS SPE, signal labels (the content of the STS SPE, including status of mapped payloads), path status, and path trace. The path overhead is found in rows 1 to 9 of the first column of the SPE.
Asynchronous Transfer Mode (ATM) is a cell-based transport and switching technology. ATM provides high-capacity transmission of voice, data, and video within telecommunications and computing environments. ATM supports a variety of traffic types, including constant bit-rate (CBR) traffic--like full-motion video and voice--where delays and cell loss cannot be tolerated. ATM also supports variable bit-rate (VBR) applications--like LAN traffic and large file transfers--where delay can be tolerated.
ATM establishes virtual connections which may be shared by multiple users. Each ATM virtual connection is identified by a combination of a Virtual Channel Identifier and a Virtual Path Identifier, referred to as a VCI/VPI value. ATM is a transport technology that formats all information content carried by the network into 53-byte cells. Since these cells are short in length and standard in size, they can be switched through network elements known as ATM switches with little delay, using what is referred to as an ATM switch fabric. Since various types of traffic can be carried on the same network, bandwidth utilization can be very high. These characteristics make the network very flexible and cost effective.
An ATM switch fabric operates to direct ATM cells from one interface to another. For a given received cell, the specific output interface of the ATM switch fabric is determined in response to a VCI/VPI value contained within the cell. Virtual channel and virtual path routing information is dynamically modified in the switch fabric as connections are established and torn down in the network. In this way the ATM switch fabric operates in response to dynamically changeable virtual connection information.
ATM cells may be encapsulated and transmitted over SONET for example using STS-1 or STS-3c, which is a concatenation of three STS-1 signals. STS-1 transports may generally be concatenated, and the combination then referred to as STS-Nc, where N is the number of STS-1 signals that are combined. In the case of STS-3c, the SPE of the resultant STS-3c frame consists of 3.times.783 bytes, together with POH. The concatenated STS-1s are multiplexed, switched, and transported as a single unit. An overhead byte of the STS-3c frame transport overhead, referred to as the H4 byte, contains an offset indicating the number of bytes between the H4 byte and the first ATM cell that is contained in the SPE.
Existing ADMs provide the optical interface to SONET, STM cross-connect matrix, and internal STM synchronization function on separate modules. In addition, fail-over capacity must be provided for each of these functions in order to support what are referred to as "protection" requirements. Such fail-over capacity is typically implemented using an "active" module that provides the function, as well as a redundant "standby" module to be employed in the event that the active board fails. This has resulted in a total of six modules used to provide the SONET interface, STM cross-connect matrix, and synchronization function.
Customers have increasingly demanded more functionality in smaller packages, for less cost. The six independent modules providing the optical interface, cross-connect matrix and synchronization function form a significant portion of the overall cost of the ADM, and together increase the form factor, power, heat dissipation, and parts and materials budgets for the device.
Accordingly there is a need for an interface design which reduces the number of boards required to provide the optical interface, cross-connect matrix and synchronization functionalities in a SONET add-drop multiplexer. The design should provide failover capacity for each of these functions to meet the protection requirements of such devices.